Ifferent methods of pitched roof construction

Pitched roofs can be built in different ways depending on the loads and sizes. This list describes some common types of pitched roof that use different methods of construction.

Couple roof A couple roof has two timbers (called rafters) that are joined at the apex at their top end and rest on the walls at their bottom end. A couple roof is used for small spans.
Closed couple roof In a closed couple roof a tie member connects the bottom ends of the rafters. The design stiffens the rafters to resist the loads on a larger span.
Collar roof The collar roof has a tie member to connect the rafters higher up the slope. The design allows the roof eaves to be lower or the ceiling to be higher.
Purlin roof A purlin roof has a horizontal timber that is supported by the external walls of the roof structure to stiffen the rafters. The purlin may be supported by struts, which rest on internal load-bearing walls.
Trussed purlin roof Trusses replace struts as support for the purlins. Trusses, which are spaced about 1800 mm apart, are braced against external walls rather than internal load-bearing walls.
Trussed rafter roof A trussed rafter roof combines ceiling joists with additional struts and ties for larger spans. The trussed rafters, which are prefabricated, are braced against the external walls. The use of trussed rafters eliminates the need for purlins and other supporting members.

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